1. Field of the Invention
The present disclosure relates to a method of manufacturing a light emitting device which employs a light emitting element such as an LED (Light Emitting Diode), particularly, to a method of manufacturing a light emitting device in which a fluorescent material etc., can be disposed uniformly, and to a spray coating machine used in the method.
2. Description of Related Art
In the general lighting market such as light bulbs for general lighting, middle and small size LCDs, large size LCDs, street lights, signal lights, and lighting for automobiles, demand for LEDs in such lighting fields is expanding with the improvement in luminous efficiency of LEDs. Typically, an LED generates white light by using an LED chip and a fluorescent material which converts a wavelength of light and is disposed around the LED chip. Examples of the method of disposing the fluorescent material include a potting method in which a liquid resin containing a fluorescent material is disposed around an LED chip and is hardened, and an electrodeposition method in which a fluorescent material is arranged around an LED chip by using an electrophoretic technique.
However, in such a potting method, a mixture of a fluorescent material and a liquid resin is applied by potting, then, the fluorescent material is allowed to precipitate spontaneously by gravity, in which, the fluorescent material is difficult to be precipitated on the corner portions and the side surfaces of the LED chip, so that a lesser thickness of the fluorescent layer may result. Such an electrodeposition method employs an electrophoretic technique, so that the fluorescent material can be attached to the corner portions and the side surfaces of the LED chip, but depends on the particle size of the fluorescent material, uneven electrophoretic behavior may occur and/or the thickness of the fluorescent material may be difficult to control.
The ratio of the wavelength-converted light by the fluorescent material varies depending on the optical path length of the light emitted from the LED chip passing through the fluorescent material layer, so that a short optical path length (i.e. a thin fluorescent material layer) increases a blue component attributes to the light emitted from the LED chip, and a long optical path length (i.e. a thick fluorescent material layer) increases a yellow component attributes to the wavelength-converted light. But in the two methods as described above, controlling of the thickness of the fluorescent material layer is not an easy task, which may result in irregular color in the emission due to disruption of a balance between the blue component and the yellow component, or may result in irregular coloration such as a yellow ring in the second optical system.
Also, generally in the case where a white light is obtained by mixing a fluorescent material in a sealing member (light transmissive resin) of an LED chip, optical output power generally tend to decrease due to dispersion of the light in a long optical path passing the fluorescent material. In recent years, a lower cost in the manufacturing is in demand for general lighting and the solution for those problems without increasing a peculiar step in manufacturing is regarded as important. Accordingly, in order to solve the problems described above, a method of forming a thin-layer of a fluorescent material has been proposed in which a coating agent containing a fluorescent material is coated on the surface of the sealing member by using a spray coating method to form a thin fluorescent material layer (see, for example, JP 2008-78659A).
In the spray coating method proposed in JP 2008-78659A, a spray-coating is applied from directly above a coating object (an LED chip encapsulated in a convex lens shape according to JP 2008-78659A) perpendicularly to the coating object, where a fluorescent material containing coating member is sprayed by using a conical swirling spray method. In which, the coating material containing a fluorescent material is sprayed in a conical swirling spray pattern to deposit a thin layer of the fluorescent material. However, in the case where a fluorescent material is applied by using the spray coating method proposed in JP 2008-78659A, due to a limitation in the spraying angle of the fluorescent material containing coating agent from the spray nozzle, uniform coating thickness is difficult to obtain particularly on the side surfaces (perpendicular surfaces of the coating objects).
Further, the spray nozzle used in the spray coating method proposed in JP 2008-78659A is placed directly above the coating object as described above. Accordingly, the coating range of the fluorescent material-containing coating agent applied with the spray nozzle on a coating object is determined generally by the machine parameters such as the diameter of the spray nozzle, the height of the spray nozzle with respect to the coating object, and the spraying pressure. Therefore, in the case where the coating object is larger than the coating range, the position of the spray nozzle and the parameters are needed to be adjusted to the coating object, so that optimum coating conditions for applying a fluorescent material on a coating object with a uniform coating at a desired thickness have not easily obtained.